Two types of 2-sided optical recording media are known. One type comprises two transparent substrates, each having a thin-film recording layer on one face. Those two faces are sealed together, preferably by a coextensive transparent adhesive layer. Instead of a coextensive adhesive, the two substrates sometimes are sealed around their center apertures and at their peripheries, thus leaving a void space between their recording layers. Such a void space results in a number of problems discussed in columns 1 and 2 of U.S. Pat. No. 4,450,452 (Ando et al.). A thin-film, oxygen-impermeable protective layer such as SiO.sub.2 may cover each recording layer as taught in the Ando et al. patent, and an identical protective layer may separate each recording layer from its substrate to inhibit oxygen and water from reaching the recording layer through the substrate. See U.S. Pat. No. 4,449,138 (Ando). As shown in the Ando '138 patent, the transparent substrate may be flat (FIG. 2) except for spiral or concentric grooves in their inner faces (FIG. 3) to facilitate tracking. Each of the transparent substrates must be nearly optically perfect and, after applying the recording and other layers, must be handled with great care under dust-free conditions until being bonded to the other substrate. When grooved, the grooves of the two substrates must be carefully aligned during assembly to insure that each set of grooves is sufficiently concentric with a spindle hole and/or a circular perimeter of the assembled substrates.
The other known type of 2-sided optical recording media has only one substrate, on each face of which is deposited a thin-film recording layer as illustrated in FIG. 3 of U.S. Pat. No. 4,331,966 (Moe). A pair of tough transparent covers such as glass layers 38' and 44' protect the recording layers. While FIG. 3 shows a void space between each of the recording layers 26' and 50 and its transparent protective cover, it would have been obvious to seal them together with coextensive, void-free adhesive layers as in the Ando et al. patent.
Because of the great care required in making and assembling the described two-sided optical recording media, they are quite expensive, although their high expense may be justified by the tremendous amount of data that can be stored in each recording layer.